INTRODUCTION TO ENVIRONMENTAL ENGINEERING Final Exam
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ENGS-37 Fall 2009 INTRODUCTION TO ENVIRONMENTAL ENGINEERING Final Exam Assigned: 10:00 am Saturday 5 December 2009 Due: 5:00 pm Wednesday 9 December 2009 Note: Please type your answers and start every problem on a new page. Rule of engagement: No communications among students. Questions may only be directed to the instructor and TAs. www.bbc.co.uk 1. (10 points) A 50-50 mixture on a molar basis of methyl mercaptan (CH3SH) and methanol (CH3OH) is being destroyed by incineration on a continuous basis. The rate of disposal is 450 kg/hour. The exhaust out of the smokestack is entirely gaseous and enters the atmosphere at a temperature of 115oC. (a) (2 points) What are the combustion reactions? (b) (3 points) What is the minimum airflow (in m3/s at 15oC and atmospheric pressure) necessary to guarantee complete combustion of both substances? (c) (2 points) What is the chemical composition of the fumes on a molar basis? [Hint: Do not forget the components of air that do not participate in the combustion.] (d) (3 points) What is the volumetric flowrate (in m3/s) of the fumes? farmenergy.org 2. (10 points) An anaerobic digester is commonly used for processing the non-recycled portion of the sludge in wastewater treatment. This generates carbon dioxide and methane, and because methane is a greenhouse gas and can be used as a fuel, the emanations from the digester are collected in a large tank. The temperature is slightly elevated because of the biological activity, and the pressure is allowed to build up until a maximum pressure is reached. Once the maximum pressure is reached, the gas mixture is drawn out and used. Consider the specific situation. A 1000 m3 tank holds a gas mixture that is 65% methane (CH4) and 35% carbon dioxide (CO2) on a volume basis at a pressure of 3 atm and at a temperature of 35oC. (a) (4 points) What mass of each gas is contained in this tank? (b) (3 points) How much more mass of methane could be held in the tank if the temperature were decreased to 25oC and the pressure remained the same? (c) (3 points) If the rate of gas production is 400 kg/day at 35oC, what is the average residence time of a gas molecule in the tank? 3. (10 points) In discussions of renewable energy, there is always mention of some primary, renewable sources of energy, such as the sun, then mention of conversion technologies, such as photovoltaic cells, to arrive at a derivative form of energy, such as electricity. Sometimes, the chain goes on, such as electricity passing through an electrical motor to generate mechanical motion. List several other energy chains, of the types: Renewable source of energy ––(conversion technology)→ Intermediate form of energy Intermediate form of energy ––(conversion technology)→ Useful form of energy Grading rule: 1 point for every correct energy type and 1 for every correct conversion technology, but minus 1 point for every incorrect energy type (such as non-renewable form of energy or not ultimately derived from a renewable source), incorrect conversion technology, and incorrect association. 4. (10 points) There are many variations of the so-called IPAT equation. One of them, for the automobile, reads as follows: Pollution units of fuel miles trips Impact = × × × × number of cars unit of fuel mile trip car (a) (2 points) In this equation, separate the technological factors, upon which engineers can improve, from the behavioral factors. (b) (2 points) Write a similar equation for buildings, with at least 4 factors. (c) (6 points) Identify two technological factors in the equation you just wrote and briefly discuss how engineers can design better buildings to lower these two factors. 5. (10 points) The performance of photovoltaic cells is projected to increase significantly in the near future, and it may be time to rethink the possibility of a solar car, that is, an electric car fitted with solar arrays on its body. Consider then a small vehicle (gross weight of 700 kg plus one 70-kg driver, not counting the batteries) that would be fitted with a 30 kW (40.2 hp) electric motor and the body of which would be covered with 35% efficient photovoltaic cells. Assume that it would be used in sunny southern California where one can count on a solar radiation of 1124 W/m2 during daytime. http://www.f9solar.com/site/?q=node/12 (a) (2 points) How much surface area of the vehicle (in m2) would have to be covered by photovoltaic cells to provide the 30 kW on an as-you-go basis? (b) (2 points) Given that the preceding answer exceeds the surface available on the vehicle body, some battery storage is required. How many kWh of electricity needs to be stored on board the vehicle if the solar radiation can be captured during seven hours of the day and the car is driven only twice a day, 20 minutes to go to work in the morning and 30 minutes to go home and run an errand in the late afternoon? How much photovoltaic cell area is now required? Is such area available on the vehicle’s exterior? (c) (2 points) For lithium-ion batteries that can hold 175 Wh/kg and with a 100% margin (i.e. doubling the storage capacity in anticipation of cloudy days and occasional heavy traffic), how much battery (in kg) is required? (d) (3 points) There is a problem, however, with the previous calculation. The weight of the battery pack is significant compared to the rest of the weight of the vehicle, and added weight to be carried around demands added power. Assuming that the power required of the vehicle is proportional to its total weight (700 kg + driver + batteries), at the rate of 37 Watts needed per kg, determine the corrected vehicle characteristics: weight of battery pack (in kg), power needed (in kW) and surface area of photovoltaic cells (in m2). (e) (1 point) Is a solar car feasible in the near future? Give at least three arguments in support of your conclusion.